Adhesive bonding processes



Feb. 2, 1954 ADHESIVE BONDING PROCESSES John J. Brophy and Robert B.Per-run, Salem, Mesa, asslgnors to United Shoe Machinery Corporation,Remington, N. 1., a corporation of New Jersey Application April 13,1953, Serial No. 348,167

This invention relates to a method of adhesively bonding parts, and moreparticularly to a method of bonding parts by activating a curableadhesive disposed between the attaching surfaces of the parts to bejoined. This application is a continuation-in-part of our applicationSerial No. 249,992, filed October 5, 1951. Present methods of adhesiveiybonding parts commonly involve the use of some form of heat for s-called activation" of the adhesive, for example, to soften adhesivecoatings on the attaching surfaces of parts to be bonded to permitcoalescence of the coatings, or to drive off a solvent where it has beenused for softening such coatings, or to initiate the setting reaction ofa thermosetting adhesive.

Vhcthcr the heat is provided by conduction through the parts or bydielectric heating with a high frequency alternating field, the adhesiveand the surrounding area of the parts must necessarily be brought to anelevated activation temperature. A large portion of the cycle of thebonding operation commonly is occupied by the period of time required tobring the adhesive up to the activation temperature. The resultingelevated temperature delays the establishment of a firm bond so that acooling-off or "dwell" period is required before stress may be appliedto the bond. In high-speed commercial bonding operations, this dwelltime may represent another large portion of the operating cycle, and itis therefore desirable to eliminate the necessity for a dwell periodfollowing activation.

Accordingly an object of the invention is the provision of a method ofbonding parts by activating an adhesive adapted to cure from an un curedcondition in which little or no adhesion exists between the parts to acured condition in which a strong bond is formed between the parts(herein referred to as a curable adhesive) in which method theactivation is immediately effective.

Another object of the invention is the provision of a method of bondingparts in which the necessity for providing a dwell period followingactivation is substantially eliminated.

Heat activation of adhesives is frequently attended by considerable riskof damage to the associated materials. For example, normally moistleather is quite susceptible of damage by heat. In dielectric heating,this susceptibility is complicated by the fact that such leather has adielectric loss factor in the same order of magnitude as, and frequentlyexceeding that of, the adhesives commonly used.

Accordingly another object of the invention is the provision of a methodof bonding parts by activating a curable adhesive in which the risk ofdamaging the parts by the activating step is greatly diminished.

12 Claims. (sing In accordance with a feature of the invention, there ispresented a method of bonding parts by activating a curable adhesivedisposed between the attaching surfaces in which the activation stepcomprises subjection of the adhesive to materialistic radiation, forexample, a beam of high-speed electrons. Materialistic radiation" asused herein, ccmists of material radiation in the sense used in Pollardand Davidson, Applied Nuclear Physics, John Wiley 8: Sons 1942, pp. 10et seq., and also X-rays and gamma rays.

In the method of our invention, little heat is evolved, substantiallythe only heat being from exothermic reaction of the adhesive, and thereis no delay in the effectiveness of the activation. Also, need for adwell period is substantially eliminated. Further, delicate orheat-sensitive materials, like leather, are substantially unharmed bythe radiation during the short exposure required for activation.

Since a bond may be formed very rapidly, the method lends itselfadvantageously to bonding operations requiring a short interval betweenactivation and the application of stress to the bond, for example, thebonding of parts only transitorily positioned or where the formation ofa bond is advantageously time-related critically with other operationsin a cycle of operations.

These and other features and advantages of the invention will best beunderstood from the following description taken in connection with theaccompanying drawings in which:

Fig. 1 illustrates diagrammatically the application of an electron beamto activate adhesive between two parts to be joined;

Fig. 2 is an enlarged sectional view through the parts shown in Fig. 1;and

Fig. 3 is a sectional view showing another form of adhesive bond betweentwo parts.

Referring to Figs. 1 and 2,. a curable adhesive ID has been disposedbetween and in engagement with the attaching surfaces of the parts 2 andI 4.

Referring to Fig. 2, a curable adhesive 10 is shown incorporated betweenthe parts I2 and H to be bonded. The parts are here indicated ascomprising leather strips. The adhesive may be applied in any convenientmanner, for example by brush application to one or both the parts at theattaching surface. In the interfacial or sandwich type of bond shown inFig. 2, the coated surfaces may then be pressed together so that theadhesive forms a connecting mass between the attaching surfaces of theparts. In the captype bond (Fig. 3) useful for Joining parts at an edge,the parts l2 and It may be held together edge to edge while the adhesiveIt is applied in a continuous mass between and in contact with the partsaround their edges. The adhesive will also 3 tend to penetrate slightlyinto the edge interfaces It, the extent of penetration depending largelyupon the viscosity and surface tension of the adhesive.

Activation of the adhesive i shown diagrammatically in Fig. 1 whereinadhesive between the parts 12 and H is illustrated as being subjected tobeam 20 of electrons fro a Eli-voltage apk siding a beam of suchelectrons is commerically available and being well known will not bedescribed here. Two well-known types which may be employed in practisingthe invention are the microwave cavity electron accelerator exemplifledby United States Letters Patent No, 2,582,186, issued January 8, 1952,on ail-application filed in the name of William E. Willshaw, and thedirect current electron accelerator exemplified .by United StatesLetters Patent No. 1,991,236, issued February 12, 1935, on anapplication filed in the name of Robert J. Van deGraaif. A commerciaiembodiment of the latter type is shown in Bulletin A-S of the h t ge Enineerin Corporation of Cambridge, Mass. Another typ of direct currentaccelerator s'lnilar to the Van de Graafi machine and which may beemployed in practising the invention is disclosed in the Journal ofApplied Physics, volume 19, July 1948,

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g'he el ctr n speed should be high enough so that the electronspenetrate through any intervenlng material, for example, part i2, and toand through the adhesive. In activating a sandwich type of bond such asthat shown in Fig. l, the beam will generally be app perpendicular tothe outer surface to provide the least absorption of the electrons bythe titer-ye" g material and hence to permit operation at minimumvoltage. The required voltage may vary widely under differentcircumstances, particularly upon the thickness and character of theintervening material. We have found voltages from about 0.4 to 3 millionvolts to be very satisfactory for activation of adhesive beneathleather, but higher or lower voltages may be used as circumstanceswarrant.

A large number of materials suitable for use as the curable adhesive incarrying out the method of the invention are available on the market.Thus a curable rubber-type adhesive may be employed as more fullydisclesed in Example IX below. We have found that an adhesive comprisinga polyunsaturated ester of the type disclosed in United States LettersPatent No. 2,155,590, issued April 25, 1939, on an application in thename 01 Benjamin S. Garvey, such as polyethylene glycol dimethacrylatemonomer or the so-called polyester resin adhesives, several examples ofwhich are given below, are quick to cure by polymerization and may beactivated by subjecting to a beam of electrons for a period of about onesecond. The minimum activation time has not been determined, but isprobably much less than one second. Because substantially the only heatevolved i the heat of reaction of the adhesive which is quickly 10st tothe surroundings, the setting of the bond is not delayed, and stress maybe applied to the bond practically immediately.

The followin examples are given for purposes of illustration to aid inunderstanding the invention and it is to be understood that theinvention is not limited to the specific materials or operatingconditions disclosed.

Example I A piece of calf leather approximately 1% inches square and 1.1mm. thick was coated on one side with an adhesive comprising an -15mixture (parts by volume) of Paraplex P-43, a polyester resin adhesivemade by Rohm and Haas, Philadelphia, Pa., and VYI-IH, a vinyl chloridevinyl acetate copolymer made by Carbide and Carbon Chemicals Corporationof New York. A similar piece or leather was placed on top of the coatedsurface and held under slight pressure while an area of the surface wasexposed to a beam of electrons from a microwave cavity type electronaccelerator of the type disclosed in the above-mentioned Patent No.2,582,186, Willshaw giving a pulsed beam of distributed velocity up to0.630 million volts but with the maximum current at 400 KEV at whichvoltage the transmission through the leather was about 1.5%. The pulselength was l 10- second and the pulse rate was 500 P. P. S. The averagecurrent density was about 0.3 microamp/cmP. The strips were held in thebeam for about 6% minutes giving an integrated exposure of about .062second (allowing a factor of .318 for the sinusoidal pulse). Uponexamination, the area subjected to the beam was found to be securelybonded.

Example II A strip of wood 1.05 mm. thick was coated on one side with anadhesive similar to that of Example I. Another similar strip was placedover the adhesive and the assembly was exposed to the same beam for 6minutes, slight pressure being applied between the strips. A good bondresulted.

Example III The procedure of Example 11 was repeated using Paraplex P-43alone as the adhesive. A good bond was obtained.

Example IV A few drops of an adhesive comprising an 80-20 parts byvolume mixture of polyethylene glycol 200 dimethacrylate monomer, anester of methacryiic acid and a glycol (polyethylene glycol 20o obtainedfrom Carbide and Carbon Chemicals Corporation of New York), and VYHHwere placed on a leather strip and subjected for two minutes to the beamfrom the cavity accelerator. The integrated exposure time was thus 0.019second. The adhesive was converted to a hard mass which adhered firmlyto the leather.

Example V The procedure of Example IV was carried out using a -10 partsby volume viscous liquid mixture of Castolite, a polyester resin made bythe Castolite Company of Woodstock, Illinois, and VYHH as the adhesive.The adhesive became hard and adhered strongly to the leather.

Example VI The procedure of Ihcample IV was carried out using each ofthe following commercial unsaturated polyester resin adhesives in turnas the adhesive: Castolite, Plaskon 920/2 made by Libbey- Owens-FordGlass Co., Toledo, Ohio, Vibrin and Vibrin 1305 made by NaugatuckChemical Co., Naugatuck, Connecticut, Paraplex P-43 and P-13 made byRohm and Haas, and Laminac 4-201 and 4428 made by American Cyanamid Co.,Plastics Division, New York, New York.

The result in each case was solidification of the adhesive and adhesionto the leather.

Example VII A strip of calf leather 1" x 4" was coated with an adhesivecomprising 85 parts by volume of Paraplex P-43 and 15 parts VYHH.Another similar strip was placed over the coated surface. the stripswere then prmsed together and exposed to a beam of electrons from adirect current accelerator of the type disclosed in the above-mentionedPatent No. 1,991,236, Van de Graaff, said accelerator being located atthe Massachusetts Institute of Technology at Cambridge, Massachusetts.This exposure was for the shortest possible time (2:1 seconds) at 1.5million volts and a current of 100 microamperes total, density about1.46 microamperes per sq. cm. Upon examination, the strips were bondedsecurely together.

Example VIII An adhesive setting composition similar to that of ExampleVII was placed along the trimmed rib and upper at the toe end of alasted mans shoe whose upper was temporarily secured in place by a toewire. The shoe was placed bottom up on a fixed speed conveyor beltassociated with the Massachusetts Institute of Technology acceleratorand thereby moved at 0.4 inch per second through a beam of 3 millionvolts and 100 microamperes for an exposure of about seconds. Uponexamination the setting composition was well activated and from thelasting viewpcnt, the upper leather, intervening liners, box foematerial and insole rib were all well secured together.

Example IX A rubber type adhesive was prepared comprising a solution in450 parts by weight of toluene. of 100 parts of the mixture:

Parts by weight Smoked natural No. 1 rubber sheet 100.0 Zinc oxide 6.0Sulphiu: 4.5 Captax (mercaptobenzothiazole) 0.5 Butyl zimate (zincdibutyldithiocarbamate) 0.25 Stearic acid 4.0

Two leather strips 4 inch wide, 3 inches long and .040 inch thicl; wereeach coated on one side with the adhesive, four coats in all beingapplied to each strip. After the last coat had dried but was stillslightly tacky, the strips were pressed together and a portion of theadhesive at one end, approximately inch in diameter, was subjected tothe beam from the cavity accelerator.

After activation, the strips were placed in the Instron tester andpulled. The standard peel pull of the unexposed portions was 3% poundsper inch while the pull of the strip at the exposed portion was 13V3pounds per inch (i. e. about 27 pounds for a l" strip activated its fullwidth).

Example X An adhesive was prepared from 100 parts by weight of GRSdissolved in 450 parts oi toluene. Two coats were applied to ,2" leatherstrips with drying of each coat for about two minutes. The strips werepressed together and a spot about in diameter near one end was subjectedto the pulsed beam from the cavity activator for an hour. The stripswere then pulled, beginning at the far end. The pull registered about 2pounds per inch pounds until the activated area was 6 reached. The pullin this area was about 12 pounds per inch.

While we have disclosed the use of an electron beam to activate theadhesive in carrying out our method, it will be appreciated that theadhesive may be subjected for activation purposes to other forms ofmaterialistic radiation in the nature of X-rays, gamma rays, protons.deuterons, neutrons, ions and alpha particles.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent of the United States is:

l. The method of adhesively bonding two parts which comprises the stepsof incorporating a curable adhesive between and connecting the attachingsurfaces of the parts, and activating the adhesive by subjecting it tomaterialistic radiation to cause the adhesive to cure.

2. The method of claim 1 in which the adhesive is curable throughpolymerization.

3. The method of adhesively bonding parts which comprises incorporatinga curable adhesive between and connecting the attaching surfaces of theparts to'be joined, and activating the adhesive by subjecting it tomaterial radiation to cause the adhesive to cure.

a. The method of claim 3 in which the material of the radiation iselectrons.

5. The method of adhesively bonding parts which comprises the steps ofincorporating a curable adhesive between and connecting the attachingsurfaces of the parts to be joined, and activating the adhesive bydirecting a beam of materialistic radiation against the outer surface ofone part to penetrate through the part into the cdh siv thereby to causethe adhesive to cure.

6. The method of claim 5 wherein the radiation is material radiation.

7. The method of claim 6 wherein the material of the radiation iselectrons.

The method of adhesively bonding parts which comprises the steps oiincorporating an unsaturated ester adhesive bet-ween and connecting theattaching surfaces of the parts to be joined, and activating theadhesive by subjecting it to materialistic radiation.

9. The method of adhesively bonding parts which comprises the steps ofincorporating an unsaturated ester adhesive between and connecting theattaching surfaces of the parts to be Joined and directing a beam ofrapidly moving atomic particles against and through one of said partsand into the adhesive to initiate a polymerization reaction thereof.

10. The method of claim 9 wherein the atomic particles are electrons.

11. The method of adhesively bonding parts which comprises the steps ofincorporating an adhesive curable through polymerization between andconnecting the attaching surfaces of the parts to be joined, andactivating the adhesive by subjecting it to material radiation forpolymerization of the adhesive.

12. The method of claim 11 wherein the material of the radiation iselectrons.

Name Date Collins Feb. 8, 1949 Arnold Nov. 13, 1951 Number

1. THE METHOD OF ADHESIVELY BONDING TWO PARTS WHICH COMPRISES THE STEPSOF INCORPORATING A CURABLE ADHESIVE BETWEEN AND CONNECTED THE ATTACHINGSURFACES OF THE PARTS, AND ACTIVATING THE ADHESIVE BY SUBJECTING IT TOMATERIALISTIC RADIATION TO CAUSE THE ADHESIVE TO CURE.